Digital terrestrial broadcasting includes, for example, ISDB-T (Integrated Services Digital Broadcasting—Terrestrial) system in Japan and DVB-T (Digital Video Broadcasting—Terrestrial) system in Europe, and these systems use OFDM (Orthogonal Frequency Division Multiplexing) method, which is one of multi-carrier transmission systems.
In the ISDB-T system and DVB-T system, transmitters periodically transmit scattered pilot signals (hereinafter, referred to as “SP signals”) which are pilot signals BPSK (Binary Phase Shift Keying)-modulated using an amplitude and phase known to receivers. Receivers sequentially estimate channel characteristics by observing the amplitude and phase of the SP signals included in received signals, and equalize the received signals using the estimated channel characteristics.
FIG. 16 is a schematic diagram showing signal arrangement of SP signals transmitted in the ISDB-T system and DVB-T system. In FIG. 16, the vertical axis indicates time in symbol units, while the horizontal axis indicated frequency in carrier units. Note that black circles are SP signals, and white circles are data-modulated signals which are modulated by transmission data.
The SP signals are transmitted after being arranged as follows: an SP signal appears every 12th carrier in each symbol; each SP signal is shifted by three carriers per symbol; and the arrangement of the SP signals is repeated in cycles of four symbols.
Next, FIG. 17 shows a structure of a conventional receiver which receives digital terrestrial broadcasting using OFDM transmission system.
In a receiver 100, broadcast waves from broadcast stations are received by an antenna 101 via paths. Then a tuner 102 tunes in to a desired broadcast wave from among the multiple broadcast waves received by the antenna 101 and converts the broadcast wave to a predetermined frequency band.
The AFC (Automatic Frequency Control) unit 103 eliminates, from the received signals input from the tuner 102, a frequency error which has occurred when the tuner 102 tuned into the broadcast wave, and outputs the received signals from which the frequency error has been eliminated, to a subsequent circuit unit. Note that a structure of the AFC unit 103 is disclosed, for example, in Patent Document 1.
The symbol synchronization unit 104 estimates a symbol timing based on the received signals input from the AFC unit 103. The Fourier transform unit 105 performs Fourier transform on the received signals input from the AFC unit 103 in accordance with the symbol timing estimated by the symbol synchronization unit 104.
The equalization unit 106 estimates channel characteristics based on the received signals input from the Fourier transform unit 105, and equalizes the received signals based on the estimated channel characteristics.
Next, FIG. 18 shows a structure of the equalization unit 106 in FIG. 17. The structure of the equalization unit 106 in FIG. 17 is the structure disclosed by Patent Document 2.
At the equalization unit 106, the received signals Fourier transformed by the Fourier transform unit 105 are supplied to an SP signal extraction unit 151 and a division unit 153.
The SP signal extraction unit 151 extracts SP signals from the received signals and estimates, using the extracted SP signals, channel characteristics of the positions where the SP signals are arranged (hereinafter, referred to as “SP signal positions”). A channel estimation unit 152 estimates channel characteristics of positions where the data-modulated signals are arranged (hereinafter, referred to as “data-modulated signal positions”), based on the channel characteristics of the SP signal positions. The division unit 153 divides the data-modulated signals by the channel characteristics estimated by the channel estimation unit 152, thereby equalizing the data-modulated signals.
Note that while details of the channel estimation unit 152 are not disclosed in Patent Document 2, in general, the channel estimation unit 152 includes a symbol interpolation filter 152a and a carrier interpolation filter 152b, the filter 152a performing interpolation processing in the symbol direction and the carrier interpolation filter 152b performing interpolation processing in the carrier direction.
A Doppler frequency estimation unit 154 observes temporal variation of the channel characteristics estimated by the SP signal extraction unit 151 and estimates the varying speed of the channel characteristics, that is to say, the Doppler frequency. An interpolation filter selection unit 156 selects a filter coefficient stored in a filter coefficient ROM (Read Only Memory) 155 in accordance with the Doppler frequency estimated by the Doppler frequency estimation unit 154.
The symbol interpolation filter 152a changes the passband width of the frequency transfer characteristics of the filter in accordance with the filter coefficient selected by the interpolation filter selection unit 152, and performs interpolation processing in the symbol direction.
Note that the narrower the passband width is, the more effectively the symbol interpolation filter 152a can eliminate noise included in the channel characteristics estimated by the SP signal extraction unit 151. Also, the slower the moving speed of the receiver is, the narrower the frequency bandwidth occupied by the variation of the channel characteristics due to the Doppler variation is. Accordingly, the slower the moving speed of the receiver is, the narrower the passband width of the symbol interpolation filter 152a can be. The equalization unit 106 effectively eliminates noise included in channel characteristics by adjusting the passband width of the frequency transfer characteristics in accordance with the moving speed of the receiver.
It should be noted that according to the arrangement of the SP signals shown in FIG. 16, the channel characteristics estimated by the SP signal extraction unit 151 are obtained only in cycles of four symbols in the symbol direction. Accordingly, when the symbol cycle is T seconds, the passband width of the symbol interpolation filter 152a cannot be made wider than 1/(4T) hertz due to the sampling theorem of complex signals.    Patent Document 1: Japanese Patent Publication No. 3074103    Patent Document 2: Japanese Laid-Open Patent Application Publication No. 2005-286636